Preparation of trimethyl phosphite and triethyl phosphite



United States Patent PREPARATION OF TRIMETHYL PHOSPHITE AND 'TRIETHY'L PHOSPHITE Jacob Rosin, Maplewood, N.J., and Oscar G. Birst en, "New York, N.Y., assignors to Montrose Chemical Company, a corporation of New Jersey No Drawing. Filed Oct. 20, 1959, Ser. No. 847,469

8 Claims. (Cl. 260-461) This invention relates to a process for producing tri methyl phosphite and triethyl phosphite.

The methods presently used for the production of these compounds are cumbersome and expensive. They are generally based on the reaction between an alcohol and phosphorous trichloride. Hydrogen chloride is produced and must be neutralized; generally, a tertiary base is used as an acceptor. The hydrochlorides formed are generally very voluminous and require dilution with a solvent to make the resulting slurry thin enough to be stirred. The volumes which must be handled are relatively large, while the necessity of recovering the base makes the operation expensive. Also, when these methods are applied to the lower alkyl alcohols, methanol and ethanol, the yields are poor.

It is an object of the present invention to provide an improved process for manufacture of a lower alkyl phosphite.

Another object of the invention is to provide a process for production of a lower alkyl phosphite in Which a catalyst is used effectively.

The invention includes other objects and advantages which will appear hereinafter wherein the present preferred practice is set forth.

The invention is based on the discovery that triarylphosphites, such as triphenyl phosphite, tricresylphosphites, trixylenyl phosphites, can be easily transesterified by methanol and ethanol. While transesterification of aryl phosphites by high boiling alcohols such as glycols, for example, is known. the transesterification of the aryl phosphites by methanol and ethanol was unexpected.

The transesterification takes place even at low temperatures, such as 25", without use of a catalyst. The rate of reaction can be greatly increased by the use of a suitable catalyst, while side reactions leading to formation of phosphonates and dialkylphosphites are reduced or eliminated. As a catalyst, one can use any alkaline transesterification catalyst such as calcium oxide, lead oxide, or the alkali metal salt of an alcohol or a phenol. The use of the sodium salt of a lower aliphatic alcohol or phenol, such as sodium methylate, sodium ethylate, sodium phenylate or sodium cresylate, is preferred because of its solubility and low cost.

Since the preparation of triaryl phosphites does not require any acceptor for hydrogen chloride, one can prepare trimethyl and triethyl phosphite in a simple manner and in practically a quantitative yield. In most of the examples below, the transesterification was run under vacuum. To be effective, the transesterification reaction is best carried on at a temperature above the boiling point of trimethyl phosphite or triethyl phosphite, respectively. If this is not observed, the concentration of trialkyl phosphite in the distillate becomes very small. At atmospheric conditions, this means that relatively high temperatures must be employed which, in turn, diminish the yield by formation of phosphonates and dia lkyl phosphites as by products. However, this does not mean that the scope of the invention is limited to sub-atmospheric pressures.

"ice The following examples illustrate this invention:

Example 1 To 373 g. (1 mole) of tricresyl phosphite (prepared from commercial cresol-xylenol mixture of average molecular weight 115), a solution of 1.5 g. Na dissolved in 25 g. of methanol were added and heated to 50 C. A vacuum of 20 mm. was then applied to the flask and 300 g. of methanol were introduced slowly at the bottom of the flask while maintaining vacuum and temperature as indicated above, and condensing the distillate. The weight of the distillate obtained was 290 g. and the assay of the distillate was 70% methanol and 30% trimethyl phosphite; this corresponds to 70% of theory based on tricresyl phosphite.

An additional yield of trimethyl phosphite can be obtained by continuing the reaction, but the concentration of trimethyl phosphite in the distillate will gradually diminish. Therefore, it is preferable tointerrupt the reaction at this point, distill off any trimethyl phosphite still present in the reaction mixture, remove free cresol by fractionation and return unconverted tricresyl phosphite residue to the next batch. The recovered cresol can be reacted with additional PCl The recovery of pure trimethyl phosphite from the distillate is described in Examples 2 and 3.

Example 2 To 310 g. (1 mole) triphenyl phosphite was added 1 g. Na. metal in 30 g. methanol. The mixture was heated to 130 at atmospheric pressure and methanol slowly introduced at the bottom of the flask while maintaining abovementioned temperature and collecting the distillate. After collecting 337 g. distillate assaying 24.5% trimethyl phosphite (corresponding to 65.7% of theory based on triphenyl phosphite), the distillation was interrupted and the phenol and uncoverted triphenyl phosphite were separated and recovered.

The trimethyl phosphite obtained in the distillate, as described in Examples 1 and 2, can be separated from the methanol by various methods. We have found that fractionation in a multiplate column is a good method provided the trimethyl phosphite is protected from decomposition. We have found that the presence of a small amount of the alkali metal alcoholate or phenylate provides stabilization of the trimethyl phosphite during distillation.

The addition of the alkali metal catalyst is also important in preservation of alcohol-trialkyl phosphite mixture from the time this is obtained as a transesterification distillate until the pure trialkyl phosphite is isolated. Without the alkaline stabilizer, the trialkyl phosphite content of the mixture diminishes relatively rapidly even at room temperature.

Example 3 Tricresyl phosphite containing 1.2% Na-cresylate as catalyst was introduued at the top of a column having four theoretical plates. The column was maintained at C. under a mm. vacuum. Methanol vapors were introduced at the bottom of the column at a rate of 72.5 weight parts of methanol per 100 Weight part of tricresyl phosphite (the latter was prepared from a commercial cresylic acid of average molecular weight of The average residence time in the column was ninety minutes.

The transesterification under these conditions was practically quantitative (98% to 99% of theory). The methanol-trimethyl phosphite vapors collected at the top of the column equilibrate at concentrations as high as 44% trimethyl phosphite and are condensed in a brine cooled condenser (brine temperature 0 or below). The eflluent leaving the bottom of the column consisted of cresol containing about methanol, between 0.3 and 0.5% trimethyl phosphite and intermediary methyl cresylphosphites and between 0.4 and 1% unconverted tricresyl phosphite. After topping oif methanol, the cresol can either be distilled or water washed and dried; it is then ready for reaction with PCl to form tricresyl phosphite.

The methanol-trimethyl phosphite fraction was dis 'tilled in a 30-plate column in the presence of 0.1% Na in the form of sodium methylate. The methanol forerun containing up to 8% trimethyl phosphite was recirculated to the bottom of the transesterification column together with fresh methanol vapors. The trimethyl phosphite was collected as the main run while a small amount of residue (methyl cresyl phosphites) was joined with tricresyl phosphite entering the top of the transesterification column.

Example 4 Ethanol was introduced into a flask containing 1 mole of tricresyl phosphite containing 1.2% Na cresylate. The system was at 75 C. under a 20 mm. vacuum. After 715 g. ethanol had been introduced, a total of 650 g. distillate containing 11.7% triethyl phosphite was obtained. After fractionating this material (protected by the addition of 0.1% Na in the form of sodium ethylate) in a 20-plate column under slightly reduced pressure, an ethanol forerun containing about 3% triethyl phosphite is obtained followed by a main run of pure triethyl phosphite.

We claim:

1. A process for producing a lower trialkyl phosphite selected from the group consisting of methyl and ethyl phosphites comprising continuously adding an alkanol selected from the group consisting of methanol and ethanol to a reaction zone containing an aryl phosphite, said reaction zone being maintained at a temperature above the boiling point of the said alkanol and the trialkyl phosphite but below the boiling point of the aryl phosphite, all boiling points being measured under the conditions of the reaction zone, and continuously removing from said reaction zone a vapor stream containing trialkyl phosphite.

2. A process for producing a lower trialkyl phosphite selected from the group consisting of methyl and ethyl phosphites comprising continuously adding an alkanol selected from the group consisting of methanol and etha nol to a reaction zone containing an aryl phosphite and an alkaline transesterification catalyst, said reaction zone being maintained at a temperature above the boiling point of the said alkanol and the trialkyl phosphite but below the boiling point of the aryl phosphite, all boiling points being measured under the conditions of the reaction zone, and continuously removing from said reaction zone a vapor stream containing trialkyl phosphite.

3. The process of claim 2 wherein the catalys't'is an alkali metal salt of a compound selected from the group consisting of alcohols and aryl hydroxides.

4. The process of claim 2 wherein the reaction zone is maintained at sub-atmospheric pressure.

5. The process of claim 2 wherein aryl phosphite is continuously added to the reaction zone.

6. The process of claim 5 wherein the aryl phosphite and alkanol are introduced into the reaction zone countercurrently.

7. The process of claim 6 whereinthe countercur'rent reaction is effected in a column having a plurality of theoretical plates.

8. The process of claim 2 wherein the said vapor stream is condensed and stabilized by the addition of an alkali metal salt of a compound selected from the group consisting of alcohols and aryl hydroxides.

Landaueret al.: J. Chem. Soc. 1953, 2224-2234. Hoffman et al.: J. Am. Chem. Soc. 78, S8175821 

1. A PROCESS FOR PRODUCING A LOWER TRIALKYL PHOSPHITE SELECTED FROM THE GROUP CONSISTING OF METHYL AND ETHYL PHOSPHITES COMPRISING CONTINUOUSLY ADDING AN ALKANOL SELECTED FROM THE GROUP CONSISTING OF METHANOL AND ETHANOL TO A REACTION ZONE CONTAINING AN ARYL PHOSPHITE, SAID REACTION ZONE BEING MAINTAINED AT A TEMPERATURE ABOVE THE BOILING POINT OF THE SAID ALKANOL AND THE TRIALKYL PHOSPHITE BUT BELOW THE BOILING POINT OF ARYL PHOSPHITE, ALL BOILING POINTS BEING MEASURED UNDER THE CONDITIONS OF THE REACTION ZONE, AND CONTINUOUSLY REMOVING FROM SAID REACTION ZONE A VAPOR STREAM CONTAINING TRIALKYL PHOSPHITE. 